The problems of foreign material deposition and corrosion and attendant affects have troubled water systems for many years. For instance, deposits tend to form on internal walls of various water systems; such as, boiler and cooling water systems, and thereby materially lessen the operational efficiency of the system.
Deposits may originate from several sources. For example, calcium carbonate, calcium sulfate, and calcium phosphate from a systems feedwater can accumulate along or around a systems metal surfaces forming scale. Also, manganese which can be found in natural waters at concentrations up to several parts per million can result in the formation of manganese deposits. Manganese oxides and hydroxides are soluble in water to 1 part per million or less and suspended solids of these compounds are commonly found. In cooling systems where the trend is towards higher cycles of concentration, the level of manganese oxides and hydroxides can increase to the point where deposition becomes a problem. Manganese occurs at significant concentrations in many well and surface waters throughout the United States and is also generally present in estuarian and coastal waters. Manganese is sometimes referred to as "Ohio River Varnish" due to its ubiquitous presence along that river and the hard, dark brown to black nature of its deposits.
An important factor effecting manganese deposition in cooling systems is oxidation to insoluble manganese dioxides by chlorination. This oxidation process is enhanced by the elevated pH which generally occurs through cycling in cooling towers.
While manganese deposition has been noted on all metallurgies, plastics and heat transfer and non-heat transfer surfaces, metallurgy can play a factor in manganese deposition. Specific stainless steel alloys, admiralty and aluminum brass have all been found to be particularly susceptible to manganese deposition and the attendant pitting of the metal.
Any waters having detectable manganese concentrations (above 0.02 ppm) may experience manganese deposition problems. At levels higher than 0.2 ppm, deposition is very likely. Manganese levels greater than 1 ppm generally cause severe problems and manganese levels rarely exceed 2 ppm even in cycled water.
Manganese deposition on tubes has been identified as manganese dioxide. Typically, it appears initially as a thin, tightly adhered, brown-black varnish which becomes thicker, rougher and more irregular if permitted to accumulate. The uniform, tightly adherent nature of the deposits lends itself well to setting up oxygen or metal ion concentration cells at holidays in the deposit probably responsible for the severe pitting effects of such deposits.
In once-through cooling systems in Japan, injection of ferrous sulfate has been credited with greatly reducing malignant impingement attack of aluminum brass caused by manganese deposition. In the United States, on-line mechanical cleaning devices as well as off-line cleaning are employed. Off-line cleaning procedures can include chemical acid cleaning, hydroblasting and metal scrapper plugs.
The present invention provides dispersants which are added to an open water system to control manganese deposition. The dispersants of the present invention also preferrably include an inhibitor to inhibit precipitation of manganese oxides. The use of the water soluble copolymer of the present invention to control calcium scale, as well as provide a passive oxide film on metal surfaces is disclosed in commonly assigned U.S. Pat. Nos. 4,659,481 and 4,659,482.